The present invention relates to a reaction force transmission mechanism for a pneumatic brake booster.
Reaction force transmission mechanisms principally comprise reaction plates which, at least at their periphery, are received in a control housing and act under pressure like an incompressible fluid. More specifically, the reaction plate establishes the balance of forces between the foot pressures exerted by the vehicle driver on the brake system and the reaction forces on the part of the brake system. Consequently, the pressure forces applied to the plate are converted into a change in shape with the limiting conditions of a constant volume and a constant pressure in the interior of the plate. A reaction plate of this type is e.g. disclosed in DE 11 92 532 A1 and basically satisfies the demands placed on conventional apparatus.
Recently, however, there is a trend to using brake boosters with transmission mechanisms which provide a panic stop function. These brake boosters are characterized by the fact that a normal brake application or an emergency brake operation with full power boosting is initiated in dependence on a predetermined relative displacement between an input member (valve piston) and a control housing that receives the reaction plate at least in part, which necessitates only low (foot) brake forces in comparison to priorb-art pneumatic-mechanic brake boosters. A brake booster of this type is e.g. disclosed in EP 901 950 A1 and comprises a pressure member which can be fixed to a control housing by way of coupling means. The comfort of actuation is, however, considered worthy of improvement because no counterforces or only low counterforces, which are not in conformity with the normal reaction force of the brake system, are transmitted to the vehicle driver in the emergency brake position.
From WO99/26826 another brake booster with a panic stop function is known which, as far as the reaction of forces is concerned, offers certain advantages, but nevertheless requires improvements with respect to the useful life of the reaction plate because the front end of the valve piston acts with a comparatively small effective surface on the reaction plate, which causes increased stress.
It has been found that the problems are overcome in that the reaction plate includes a recess in the area of the abutment of the valve piston""s extension, and in that a substantially incompressible, elastic insert is placed into the recess. This measure permits reducing stress, in particular shearing strains, in the inside of the reaction plate and, consequently, increases the service life of the reaction plate.
It is advantageous in another embodiment of the present invention that the valve piston""s extension is configured as a separate tracer pin, and the diameter of the tracer pin in the area of its abutment on the reaction plate corresponds to the diameter of the recess. Thus, the radial outside part and the insert are separated in the area of the most intense (shearing) strains.
Finally, it is favorable that the insert and the outside part are made of materials with a different elasticity module (or different Shore hardness). This permits adapting the brake force booster with respect to the switch-on threshold of the panic stop function.
Another favorable effect is achieved by that, starting from the abutment surface of the output member (push rod) on the reaction plate side, an axial recess is provided into which the insert is additionally slipped. Thus, the circular-cylindrical insert (in the non-deformed condition) has a length greater than the thickness of the reaction plate. This imparts a greater volume to the insert and allows the insert to transform pressure stress into deformation work to a still greater extent.